This invention relates to an apparatus by means of which the flow rate of a gas can be measured stably and continuously for a prolonged period of time in a high temperature and dust containing environment.
In measuring the flow rate of a fluid flowing through a duct, it is known to use the following relation: a quantity indicative of the total energy possessed by a fluid (`total pressure`) is equal to the sum of the energy proper to the fluid in the static state (`static pressure`) and the energy proper to the fluid in motion (`dynamic pressure`); and it has been known to make use of the above described relation to measure the total pressure and the static pressure of the fluid to find the flow rate on the basis of the difference between those pressures.
Also, in measuring the flow rate of a fluid in a duct, there are few portions of the duct that lend themselves to optimum measurement of the flow rate, the rate of flow differing from one zone within a cross-sectional area of the fluid within the duct to another. For reducing the measurement error caused by these different flow rates across the cross-sectional area of the fluid, it is the conventional practice to divide the circular cross-sectional area of the duct into concentric circles having equal cross-sectional areas for each of which the total pressure of the fluid is measured and these measured values of the total pressure are averaged for calculating the approximate total pressure, as specified in JIS (Japanese Industrial Standards).
In a conventional apparatus for measuring the flow rate of a high-temperature dust-containing gas in accordance with the above principle, the service life of the apparatus is drastically reduced by the combination of the following factors.
(i) Pressure measuring ports and pressure conduits are repeatedly stopped up with dust and dirt solid particles introduced into the fluid pressure measurement unit with the gas.
(ii) Dust and dirt are deposited on the pressure measuring ports and the pressure conduits because the measurement unit is heated to a high temperature.
(iii) The measurement unit suffers from considerable chemical attack due to the high temperature atmosphere so that its service life is severely reduced.
(iv) The hardness of the component material of the measurement unit is reduced due to the high temperature environment so that physical erosion of the unit by the dust contained in the gas increases.